Stabilized wet process phosphoric acid

ABSTRACT

Wet-process phosphoric acid products substantially free of post precipitation.

This is a continuation-in-part of my copending application Ser. No. 951,804, filed Oct. 16, 1978, now abandoned, which is a continuation of my Ser. No. 767,175 filed Feb. 9, 1977, now abandoned, which is a division of my Ser. No. 683,756 filed May 6, 1976, now abandoned. Ser. No. 951,804 is also a continuation-in-part of my application Ser. No. 812,319, filed July 1, 1977, now U.S. Pat. No. 4,110,422, which is a continuation-in-part of said Ser. No. 683,756. This is also a continuation-in-part of my copending application Ser. No. 883,381, filed Mar. 6, 1978, now U.S. Pat. No. 4,164,550, which is a continuation-in-part of Ser. No. 812,319, aforesaid. A related case is my copending Ser. No. 54,446, filed July 3, 1979, now U.S. Pat. No. 4,248,846, "Improved Production of Stabilized Wet Process Phosphoric Acid". The aforesaid Ser. Nos. 951,804, 883,381, and 54,446, now U.S. Pat. No. 4,248,846; and U.S. Pat. No. 4,110,422 are each herein incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

In conventional methods for the production of wet process phosphoric acid, finely ground phosphate rock is reacted with dilute phosphoric acid (10% P₂ O₅ to 25% P₂ O₅) and sulphuric acid which may or may not be diluted. The reaction product is leached (i.e., digested or reacted with the acid) to yield a crude aqueous phosphoric acid solution in which is suspended a substantial quantity of solid impurities. This slurry is filtered to separate most of the undissolved gypsum and other solid impurities to yield a crude (22% P₂ O₅ to 35% P₂ O₅) aqueous product sometimes known as the number one filtrate. This filtrate contains suspended, finely divided, impurities that were not removed during the solid-liquid separation step plus solids that have crystallized after filtration. The solution also contains dissolved impurities. Reference is made to Chapter 16 of Volume II, Phosphorus and its Compounds, Van Wazer, Interscience Publishers (1961).

The problems occasioned by suspended and dissolved impurities are particularly acute with respect to wet process phosphoric acid. Variable quantities of impurities in the phosphate rock such as organics, calcium, potassium, sodium, aluminum, iron, strontium, titanium, silicon, uranium, vanadium, fluorine, magnesium, etc. are put into solution during the reaction of the phosphate rock with sulphuric and/or phosphoric acid. Standing, cooling, or concentration of the phosphoric acid solution results in additional solids being formed and precipitated to form a sludge consisting mainly of complexed iron, aluminum, potassium, sodium, calcium, etc.; and phosphates, fluorides, silicates, sulphates, etc. This is known in the art as post-precipitation.

Concentrated wet process phosphoric acid, upon standing or during shipment in tank cars or the like, deposits a layer of solid matter forming sludge as above described, which renders handling stored or shipped acid exceedingly difficult and frequently economically unfeasible. Some solids may be removed from the crude solution by conventional filtration or centrifugation. With respect to conventional filtration, however, the concentrated acid is very viscous and difficult to filter. Further, the solids are of such nature that they readily blind the filter cloth and the solids are difficult to remove from the filter. With respect to conventional centrifugation, many of the finely divided solids will not be removed, particularly the organic particles, which are lighter and are not removed with the larger heavier inorganic solids from the solution.

SUMMARY OF THE INVENTION

This invention relates to phosphoric acid products which may be stored for long periods or shipped long distances without appreciable post-precipitation. In accordance with the present invention it has been discovered that an eminently satisfactory clarified, stabilized, concentrated phosphoric acid can be achieved by means of a series of specific but interdependent steps.

To produce the products of this invention I start with a clarified filtrate. If the available No. 1 filtrate contains suspended solids, it must first be clarified. Crude phosphoric acid from the conventional wet process acid filter at 22% to 35% P₂ O₅ concentration (No. 1 filtrate) ordinarily contains 0.5-8.0 wt. % suspended solids and accordingly is first substantially clarified. In this step it has been found satisfactory to remove 85% to 98% by volume of the suspended solids. These solids may be returned to the digestion circuit or to the phosphoric acid filter feed tank.

The clarified acid with less than 2% by volume of suspended solids is next mixed with an aluminum silicate material, preferably perlite, e.g., perlite filter aid, then passed to a conventional wet process phosphoric acid evaporator. After reaching a concentration of 48% to 50% P₂ O₅ the evaporator product is introduced to a crystallizer, then subjected to settling where a small portion is removed as underflow containing approximately 10 to 40% by weight of solids. This underflow may be returned to the 30% filter feed tank or to diammonium phosphate, run-of-pile triplesuperphosphate, or granular triplesuperphosphate circuit, or it may be separated by centrifugation or filtration to recover the P₂ O₅ values. The overflow preferably goes to another evaporator and is brought up to 52 to 63% P₂ O₅, preferably 60% P₂ O₅. This product may be immediately loaded in a tank car or stored in tanks until ready to be shipped or used.

In the drawings,

FIGS. 1, 2, and 3 show in schematic form flow sheets for several embodiments of the invention.

FIG. 1 is a simplified flowsheet for a hypothetical large scale commercial plant, for making 1000 tons per day of 100% P₂ O₅, as contained in the final product, which is phosphoric acid concentrated to 60% P₂ O₅.

FIG. 2 is basically like FIG. 1 except that the quantities have been reduced to pilot plant size, and certain modifications have correspondingly been made (e.g., one evaporator is used for two stages of concentration).

FIG. 3 is also a flow diagram for a pilot plant, again using one evaporator, but with additional flow options, including a preferred type of crystallizer.

FIG. 4 is a simplified flow diagram for a commercial-size plant used in making the products of the invention.

FIG. 5 shows a tank car and a layer of precipitate laid down by acids of the prior art.

GENERAL DESCRIPTION

To make the products of this invention the steps in order are:

1. Clarification in the 22 to 35% P₂ O₅ range, if an already clarified dilute phosphoric acid is not available.

2. Addition of aluminum silicate material, e.g., perlite or other aluminum silicate suitably in company with small amounts of sodium and potassium compounds, all finely divided so as to be soluble in phosphoric acid as the concentration is increased from approximately 22-35% P₂ O₅ to approximately 42-52% P₂ O₅. Perlite is a naturally occurring mineral, being a type of volcanic glass, and is available commercially in various grades. The product is an aluminum silicate with varying amounts of sodium and potassium silicates.

Typical analyses of suitable grades of perlite preferred for use in this invention are shown below. The first column gives the analyses of the filter aid grade of perlite used in the examples in this specification. The second column gives a weight range of perlite components, suitable for use in this invention.

                  TABLE 1                                                          ______________________________________                                         Suitable Grades of Perlite                                                                        Col. 1, Col. 2,                                             Component          Wt. %   Wt. %                                               ______________________________________                                         Silica (SiO.sub.2) 73.41   67.05-74.82                                         Alumina (Al.sub.2 O.sub.3)                                                                        12.34   10.82-18.55                                         Iron Oxide (Fe.sub.2 O.sub.3)                                                                     1.33    0.25-3.19                                           Magnesium Oxide (MgO)                                                                             .1      .01-.70                                             Calcium Oxide (CaO)                                                                               .75     0.78-3.8                                            Sodium Oxide (Na.sub.2 O)                                                                         2.95     1.2-4.15                                           Potassium Oxide (K.sub.2 O)                                                                       5.33    2.24-5.6                                            Phosphorus Oxide (P.sub.2 O.sub.5)                                                                        0.01-0.12                                           Manganese Oxide (MnO.sub.2)                                                                               0.02-0.04                                           ______________________________________                                    

The preferred varieties of perlite contain, before expanding and milling, 2-6% trapped H₂ O and a Na₂ O:K₂ O weight ratio in the range of about 1/11/4. Material of this type, after expanding and milling dissolves readily in the acid mostly during the evaporation step. Very little seems to dissolve on simple mixing. The ability of the perlite or other aluminum silicate material to dissolve in the acid is regarded as essential in the practice of this invention. Of the available grades of perlite, I have found that two are particularly advantageous in carrying out my invention. One of these grades is filter aid grade. The other is light weight plaster aggregate grade, which should be crushed for this use. Both forms are available commercially. Both are made by "popping" mineral perlite in a furnace, i.e., heating it at about 1500°-2000° F. so that the contained water forms bubbles in the softened glass, whereby the particles are expanded to 15 or 20 times their original volume.

I can add 1-40 lbs. of perlite or other aluminum silicate material per ton of equivalent P₂ O₅ contained in the acid, or 0.05-2.0 wt. % based on the P₂ O₅ in the acid. All tons herein are short tons (2,000 lbs.).

Continuing now with the steps of the process:)

3. Concentration to 42-52% (preferably 48-50%) P₂ O₅ acid.

4. Removal to a crystallizer where the fresh 42 to 52% acid is continuously fed to a recirculating acid volume.

5. Sedimentation of the crystallized product and removal of a portion of the underflow.

6. Preferably concentration of the overflow to 52-63% P₂ O₅ product.

Based on pilot plant operation it has been found that the preferred method of clarification, step 1 above (where necessary) is one of settling under specific conditions. Filtrate from a conventional filter is pumped to the head box of a mixing launder, where a polymeric flocculant is also added. Polyhall M295, a polyacrylamide-type flocculant commercially available from Stein, Hall & Co., Inc., of New York, N.Y., has been successfully used at a wt. concentration in water of 0.085%. This concentration has been varied from 0.02 to 0.4% by wt. in water without noticeably affecting the results. The Polyhall solution is added to the filtrate at a rate of 0.2% to 3.8% by volume of the 22% to 35% filtrate preferably 0.8% by volume.

Numerous other flocculants are available for clarification. Examples include water soluble high molecular weight synthetic polymers, guar, etc.

The polyacrylamides and the hydrolyzed polyacrylonitrile resins and their salts and derivatives are particularly useful. Flocculants are used conventionally in the phosphoric acid industry in settling higher concentrations of acid, e.g., 42%, 52% and 54%. Such flocculants are available commercially and are useful in this invention.

A temperature drop of the filtrate of 1° C. to 20° C. is beneficial to proper clarification. The feed to the clarifier has been found to be satisfactory from 30° to 72° C. with best results obtained between 46° C. to 54° C. The temperature drop was best accomplished in a launder which has partial baffles so the polymer-filtrate mixture must follow a zig-zag path before discharging to a stilling well in the clarifier. In the pilot plant for 5 to 10 GPM (cf. Example 3) the launder was 6" wide with sides 6" high and 8"-0' long sloped 2 on 12 or about 91/2° off the horizontal, having 10 baffles 4" wide by 4" high and alternately attached to the launder sides. This provided a gentle but thorough mixing and cooling action. A mix tank can be used, but care must be taken to stir very slowly, so as not to disintegrate the flocs.

The pilot plant clarifier of nominal 50 sq. ft. has been able to satisfactorily operate up to 10 GPM. Nominal rates have been 6 GPM to 8 GPM or 81/3 sq. ft. per GPM to 61/2 sq. ft. per GPM. Satisfactory results have been achieved at a rate of 5 sq. ft. per GPM. Since phosphate rock is a mineral found in nature, its composition is not always uniform. It has been found by experience that some dilute acids produced may require up to 20 sq. ft. per GPM for acceptable clarity.

Experience has also shown that an underflow rate of 2 to 25% by volume of the feed is desirable. For instance for a nominal clarifier feed rate of 6 GPM containing 1 to 3% by wt. of suspended solids, the underflow rate was 0.48 GPM or about 8% by volume of the feed.

The retention time in the clarifier, based on 6 GPM feed rate to the clarifier was about 73 minutes. Good results have been obtained with retention times as low as 43 minutes.

Underflow from the clarifier may be filtered and washed and the solids discarded. Tests have shown that the filtration rate of the underflow was in excess of 100 gallons per square foot per hour, and the cake washes readily with water. This cake averages 25 to 45% moisture and on a dry basis contained less than 4% P₂ O₅. In the pilot plant the underflow was returned to the conventional filter.

Depending on the grade of rock used for digestion in the conventional acid plant there are times when a small amount of the flocculated material will float rather than sink. To overcome this a baffle is placed in front of the overflow weir in the clarifier and a skimming device such as shown on page 19-50 of Chemical Engineers Handbook, Perry, Chilton and Kirkpatrick, 4th edition, McGraw-Hill Book Co., N.Y. This float material after skimming is combined with the underflow, outside of the clarifier, for ultimate disposal.

A typical analysis of the clarifier feed after having been mixed with the polymer solution on an as is basis is given in Table 2 below (following Example 3) as "Clarifier Inlet".

Typical analyses of the clarifier underflow and overflow on an as is basis are shown in the same Table.

The clarifier overflow is next mixed with an aluminum silicate material, preferably a good commercial grade of perlite filter aid. Such material, e.g., perlite is added at a rate of 1 to 40 lbs. per ton of P₂ O₅ (0.05-2.0 wt. %), generally 5 to 15 lbs. perlite per ton P₂ O₅ (0.25-0.75 wt. %) but more preferably 6 to 8 lbs. per ton P₂ O₅ (0.3-0.4 wt. %). After mixing, the perlite-acid mixture is fed to a conventional evaporator. The product of this evaporation may be taken off at 42% P₂ O₅ or higher although 46 to 50% P₂ O₅ is the preferred range, however, even up to 52% is not detrimental. The most preferred range, however, is 48 to 50% P₂ O₅.

After this first evaporation the evaporator product is introduced to a crystallizer. Here the incoming acid is preferably mixed with acid that has had at least 30 min. and even more preferably 1 to 3 hours recycling in a crystallizer provided with a recycle tube and cone tank. The ratio of recycle acid to incoming feed is best at 1.1 to 30:1. The cone is tapered and equipped with rakes to discharge the acid-crystal product from a centrally located annular opening at the bottom. Recycling is accomplished by bubbling air within the inner tube causing an air lift, or by a propeller stirrer or both.

The product of the crystallizer is subjected to gravity settling for a period of time sufficient to allow the heavier crystals to settle out. These settling crystals form a slurry of about 25% by wt. of solids. This slurry which is approximatley 1 to 20% by volume of the crystallizer feed, and generally 2 to 4% by volume, then may either be used to make dry products such as diammonium phosphate, triple superphosphate, or run-of-pile triple superphosphate.

The settler overflow can be accepted as final product. However it is next preferably evaporated to 52-63% P₂ O₅ concentration, and preferably to about 60%. This product, which is now clarified and stabilized, may be shipped directly or sent to storage tanks until ready for use. Some additional solids may come out because of cooling; however, these solids are generally less than 1/2% by wt. and do not cause excessive post-precipitation.

Post precipitation in wet process phosphoric acid has always been a problem. According to Encyclopedia of Chemical Technology, 2nd Edition, Vol. 9, p. 87:

"When wet-process acid is to be used in another process in the same plant operation, a relatively high content of impurities can be tolerated. But when it is to be shipped away from the point of production, for use as a merchant material or in some other plant of the producing company, the solid impurities are removed to the extent that problems in shipping and handling are reduced. The major constituent of most sludges containing from 52 to 54% P₂ O₅ acid is a complex salt with the composition (Fe,Al)₃ KH₁₄ (PO₄)₈.4H₂ O. Its composition is quite uniform except for variations in the iron/aluminum ratio. The extent of formation depends on the amount of potassium in the rock; a small amount can precipitate a major part of the iron and aluminum because the compound contains only about 4% potassium. Precipitation continues over a long period in storage until practically all the potassium is removed. Other dissolved impurities such as sulfate, chloride, and fluoride affect the rate of precipitation." [Emphasis supplied.]

Accordingly K-containing compounds may be contraindicated as additives for the prevention of post-precipitation. Hence it is surprising that perlite, which contains potassium, is effective for this purpose. And, as a matter of fact, as shown in Table 2 below (following Example 3), my final product contains a sizeable amount of K₂ O. Nevertheless post-precipitation is inhibited to a substantial extent.

EXAMPLE 1

This example has not actually been carried out, but is based on my information and belief, and especially on the results of my work in a small pilot plant. (For actual commercial plant runs, see FIG. 4 and description.)

Reference is made to FIG. 1. Based on a hypothetical production of 1000 tons per day of 100% P₂ O₅, as contained in 60% P₂ O₅ phosphoric acid, the quantities noted apply. The feed stream is No. 1 filtrate analyzing about 29.3% P₂ O₅. The quantity of feed is 1100 tons per day P₂ O₅, equivalent to about 480 gallons per minute with a specific gravity of about 1.30. This No. 1 filtrate enters via line 1 into a mixing vessel 2. There is also added to the mixing vessel 2 a flocculant solution. The particular flocculant is a polyacrylamide derivative available commercially as "Polyhall", from Stein, Hall & Co., Inc. This flocculant is prepared as an aqueous solution, 0.085% weight of flocculant in water. This aqueous solution of flocculant is added via line 3 to mixer 2 as a 1% by volume of feed, i.e., 4.8 gallons per minute of flocculant solution. This is equivalent to 57.6 lbs. per day of dry flocculant. The mixture of No. 1 filtrate and flocculant is then passed via line 4 to clarifier 5. Within the clarifier, the flocculant aids in the rapid and thorough settling of suspended solids contained in the No. 1 filtrate. These solids are typically 3.0% of the total weight of the No. 1 filtrate. Two streams are drawn from the clarifier, namely the underflow at line 6 and the product stream at line 7. The underflow at line 6 approximates 8% by volume of the feed going into the clarifier. Thus this underflow exits the clarifier at a rate typically 38 gallons per minute of slurry, equivalent to 338 tons per day of slurry having a specific gravity of 1.47. This stream is returned to the filter. By filter is meant the main filter separating gypsum from the No. 1 filtrate. The underflow comprises about 25 wt. % solids which analyzes about 4% P₂ O₅ by weight on a washed dry basis.

Returning now to product stream 7, this exits the clarifier at the rate of 1026 tons per day of P₂ O₅, equivalent to about 440 gallons per minute of liquid with a specific gravity of about 1.33. This stream proceeds to mixing tank 8, where perlite is added at the rate of 11,000 lbs. per day of perlite, or about 10 lb./ton of P₂ O₅ in the incoming stream. This material is thoroughly mixed with the liquid in tank 8. A production stream is drawn off through line 9 continuously from mixing tank 8. This stream has very nearly the composition of incoming stream 7, to wit, 1026 tons per day P₂ O₅, 440 gallons per minute, specific gravity 1.33. This stream now proceeds to evaporator 10, where it is heated to 85° C. under an absolute pressure of 4-6 inches of mercury, to evaporate the product to approximately 50% P₂ O₅. This product exits the evaporator 10 via line 11 to crystallizer 12. The product entering crystallizer 12 flows at the rate of 1026 tons per day P₂ O₅, 210 gallons per minute, and has a specific gravity of about 1.63. The crystallizer 12 provides 2 streams, an underflow stream at 13 and a product stream at 14. The underflow stream at 13 is about 2% by volume of the feed. Thus it is about 4.2 gal. per minute of slurry or 43.7 tons per day of slurry. It has a specific gravity of about 1.73. Stream 13 contains sufficient P₂ O₅ to warrant recovery of its P₂ O₅ content. Stream 13 comprises about 21% by weight of solids and about 79% by weight of liquid recoverable as filtrate. The solids comprise 4.3 tons per day of P₂ O₅. This high content of P₂ O₅ precludes returning underflow 13 to the primary filter, since it would be irrevocably lost with the gypsum. Hence it should be sent to various recovery systems such as run-of-pile, or granular triple superphosphate, for plants having such facilities. The liquid portion of underflow 13 comprises 14.9 tons per day of P₂ O₅ containing 46 to 50% P₂ O₅. If underflow 13 is recovered in the manner recommended it would, of course, recover both solids and liquid, and hence recover all of the P₂ O₅.

Returning now to product stream 14, this exits the crystallizer at the rate of 1002 tons per day of P₂ O₅ equivalent to 206 gallons per minute, having a specific gravity of about 1.62. This stream goes to a second evaporator 15, where it is concentrated to a product containing about 60% P₂ O₅ by heating to a temperature of about 85°-90° C. and under a vacuum of about 1-2 inches of mercury, absolute pressure. In this second evaporator there may be expected some very small loss of P₂ O₅ in the condenser. The product exits the evaporator via line 16 and proceeds to storage tank 17. Storage tank 17 is preferably equipped with means to draw off any sludge that collects on the bottom. In the latter manner about one ton per day of P₂ O₅ (as contained in the 60% P₂ O₅ acid) is drawn off through line 18. The final effluent line is shown at 19, and through this line there is available 1000 tons per day of P₂ O₅ as 60% P₂ O₅ wet process phosphoric acid.

To convert the foregoing quantities to a pilot plant scale the quantities given are simply reduced by a factor corresponding to the size of the pilot plant. For example, I used a pilot plant with varying capacity, but nominally about 11.6 tons per day of P₂ O₅, as 60% P₂ O₅ acid. Thus all quantities were divided by a factor of 86. This means that the acid feed flowing through line 1 was at the rate of 12.8 tons per day P₂ O₅, or 5.57 GPM. Overflow from the clarifier was typically 11.9 tons per day P₂ O₅, and underflow was typically 0.44 GPM. Perlite was added to the mixing tank 8 at a rate typically 128 pounds per day. Feed from the mixing tank 8 to the evaporator 10 is typically 11.9 tons per day P₂ O₅ of clarified acid, or about 5.1 GPM. Product leaving evaporator 10 is likewise about 11.9 tons per day P₂ O₅, at about 2.44 GPM. Underflow from crystallizer 12 is typically 0.05 GPM. Overflow is typically 11.63 tons per day P₂ O₆ at the rate of about 2.39 GPM. The overflow product goes to evaporator 15, and here the effluent provides about 11.63 tons per day P₂ O₅, collectible in storage tank 17 as the final 60% product.

EXAMPLE 2

This embodiment differs in certain respects from that described in Example 1. The main difference in the equipment was the use of one evaporator in lieu of the two evaporators described in Example 1. In this example, for the sake of economy, one evaporator was used in the pilot plant for both concentration steps. In the first evaporation step 29% acid was concentrated to acid analyzing about 50% P₂ O₅. This product was then collected and run back through the evaporator to concentrate it to 60% P₂ O₅ acid. There are various ways of doing this. One system is shown in FIG. 2. In FIG. 2 it will be noted that the same equipment of FIG. 1 is retained, except that evaporator 15 is omitted and a 30% acid feed tank 27 is added. Referring again to FIG. 2, one evaporator can be utilized in the following way. 30% acid is run into feed tank 27. This is a holding tank with a capacity of 4000 to 6000 gallons. This acid exits via line 28 into a small surge tank 29 of about 200 gallons capacity. Operations then proceed substantially as described in Example 1 through mixer 3, clarifier 5, mixer 8, and thence to evaporator 10. Here the product is evaporated to 50% P₂ O₅ acid as in the preceding example, passes into crystallizer 12, and thence into storage tank 17, where it is collected. At this point valve 9a is closed, and the 50% acid is pumped from storage tank 17 back to evaporator 10, where it is next concentrated to 60% P₂ O₅ acid. Following this concentration, it exits the evaporator there via line 26 through open valve 26a, thence to storage tank 17. There are of course numerous alternates to this scheme for using one evaporator instead of a plurality of evaporators. A further modification involves the use of two separate storage tanks, one for the acid first concentrated, e.g., to 50% and the other for acid as concentrated a second time, e.g., to 60% P₂ O₅. The lines are appropriately valved to avoid backflow and improper mixing.

A curious effect of the use of one evaporator for two evaporations was noted. During the first evaporation step, where 29-30% acid was evaporated to 50% acid, some scaling was noted on the evaporator walls. During the second evaporation, where the 50% acid was concentrated to 60% acid, this scaling was completely removed.

EXAMPLE 3

Reference is made to FIG. 3. Primary filtrate flows through line 35 into storage vessel 27. This vessel is conveniently 4000-6000 gallons capacity. Product from this storage vessel is pumped via pump 36 and line 28 into surge tank 29. The latter has about 200 gallons capacity and is heated by steam coils to about 47° C. The product is then pumped via line 37 and pump 38 followed by line 39 to launder 65. Meanwhile a solution of flocculant is mixed with water in vessel 40, and it is pumped therefrom via line 41, pump 42, and line 3 into the launder 65. The two streams mix in the launder 65 and are discharged into the stilling well 41 of clarifier 5. Sludge exits the clarifier at 6 and is sent to the filter. Overflow exits the clarifier at 42 and is discharged into vessel 8, where it is mixed with perlite. Vessel 8 is conveniently equipped with a mixing propeller 43 and means for steam heating shown schematically at 44. The perlite is mixed at 66 into the liquid, which is discharged from vessel 8 via line 44 and pump 45 through lines 46 and 61 where it enters evaporator 10. The acid is initially concentrated in evaporator 10 with the aid of vapor box 47. Water vapor exits via line 48. In practice, line 48 goes to a conventional condenser (not shown), where it is cooled and condensed, and exits the system as liquid via a conventional barometric leg (not shown). Liquid condensate in the vapor box divides into two streams. One stream shown at 49 recycles back to the evaporator 10 via pump 50. The second stream is taken off the vapor box at line 50 where it discharges into evaporator product tank 51 as approximately 50% P₂ O₅ acid. The overall evaporation system used in this Example is known as "f.c." ("forced circulation") and is conventional in the art. From tank 51 the product exits via line 52, pump 53, line 54, and line 55 to crystallizer 12. Underflow from crystallizer 12 exits via drain 13 and is utilized for its P₂ O₅ value as herein described. Overflow from crystallizer 12 exits via line 14 into tank 56. This tank is conveniently equipped with propeller mixer 57. Product exits the tank via line 58. It may be collected as the final product at this point via line 59, or it may be further concentrated by returning it to evaporator 10 via lines 60 and 61. After it is once more concentrated, e.g., to 60% P₂ O₅, it is again collected in evaporator product tank 51 but this time it is sent via line 54 and line 62 to 60% product tank 63, where it is available for final shipment and/or use. This product exits the evaporator at 85°-90° C. As it starts to cool, a very small and harmless amount of solids may precipitate.

The entire system is adequately equipped with valves for control of flow direction in pertinent lines, e.g., valves at 60a, 60b, 61a, 59a, 13a, 55a, 28a, 50a, etc.

In one variation, evaporated product can be returned via lines 58, 60, and 67 to perlite addition vessel 8. To do this, valves 60a and 67a are open; valves 59a and 60b are closed. Also, if for any reason additional perlite is to be added before this product goes through the evaporator the first time, this can be done by closing valve 61a and opening valves 60b and 67a. In essence, this simply recylces the product out of and back into vessel 8.

A mixing tank 40 had a capacity of 211 gallons. It was found that this amount of flocculant solution lasted about one week during normal operation of the pilot plant.

On occasion it is convenient and desirable to add soda ash to the incoming No. 1 filtrate, e.g., to vessel 29. When this feature is employed, the amount of soda ash is conveniently 0.1 to 2.0% by weight of the content of P₂ O₅.

Soda ash causes precipitation of Na₂ SiF₆ which in turn helps to sink the flocs.

The aforesaid pilot plant was run semi-continuously (i.e., continuously but with several shutdowns) over a period of some weeks, and the product was used to fill three tank cars.

The values given in Table 2 are typical average values, but, of course, are subject to variation from run to run.

THE EQUIPMENT

Generally speaking all equipment used in the pilot plant described in FIG. 3, including variations within the pilot plant during the course of filling eight tank cars, was of conventional design.

THE CLARIFIER

The clarifier was cylindrical in section with a conical bottom leading to an underflow drain. It was equipped with a stilling well and with rakes generally conforming to the conical bottom and driven by an electrical motor the shaft of which was axial with the center line of the stilling well. The incoming feed from the open trough launder is added to the clarifier through the stirring well in order to minimize turbulence. The clarifier is best described as to capacity as being 6-8 square feet of surface per gallon per minute of feed. This capacity may vary somewhat depending on the type of feed. This again may vary to some extent on the type of reaction system, starting with phosphate rock, etc. In one operation a feed was used where it was necessary to use a clarifier capacity of about 20 square feet/GPM. However 6-12 square feet/GPM is generally suitable.

The clarifier operates at its most efficient when full and when operated on a continuous basis.

The clarifier may be equipped with conventional accessories, e.g., skimmer, overflow weir, etc.

In clarification, the suspended solids that are removed consist mainly of CaSO₄.2H₂ O, Na₂ SiF₆, K₂ SiF₆, and organic materials.

Although flocculation clarification was used in the pilot plant, alternate procedures and equipment are available and are well known to those skilled in the art, e.g., the Lamella settlers, which are a series of parallel spaced-apart plates, somewhat tilted, with very large theoretical settling surface per volume of liquid. Even filtration can be used.

EVAPORATOR

The evaporation in FIG. 3 had a feed rate of 3 to 5 gallons per minute. It was steam-heated, and operated under a vacuum of 24-26 inches (4-6 inches of mercury absolute pressure) in evaporating 29-30% acid to 50% acid, and at 1-2 inches of mercury in concentrating that acid to 60% P₂ O₅ acid. (Actually, acid analyzing 62-63% P₂ O₅ was made during some runs of the pilot plant.) It was equipped with a vapor head box for venting vapors via a conventional system.

When the acid is to be shipped away from the plant site, it is preferably concentrated to about 60% P₂ O₅ to save freight. Otherwise freight has to be paid on the extra water. Concentration to 60% is a good practical upper limit when heating the evaporator with steam. Higher concentrations can be achieved by heating with Dowtherm. If the buyer wants a more dilute grade, he simply adds water.

In using this process in a commercial-size plant the evaporators would be, of course, much larger and with many refinements that would not ordinarily be involved in a pilot plant operation. Commercial evaporators are well known to those skilled in the art and have been described in the standard reference works on phosphoric acid. See, for example, Van Wazer, supra, pp. 1046-7; and 2nd Edition, Encyclopedia of Chemical Technology, Vol. 1, page 93.

During evaporation some SiF₄ is stripped off, and provision can be made for its recovery by conventional methods.

CRYSTALLIZER

Various embodiments were used, and all were found to be generally satisfactory. The one that seemed to perform best was a so-called Pachuca type of settler-crystallizer, shown schematically at 12 in FIG. 3. This was generally cylindrical in cross-section with a conical bottom and was equipped with an internal cylinder again terminated with an open conical bottom. A third cylinder open at top and bottom was positioned within the second shell. The outer and middle shells were equipped with rakes generally conforming to the contour of their respective conical bottoms, and driven by an electrical motor and shaft on the center line of the innermost cylinder. The liquid feed product was added into the innermost cylinder along with air to provide aeration. The air was not necessary to prevent post-precipitation in the final product, but did aid in internal liquid recycling, control of scaling, and in the formation of better crystals, within the crystallizer.

The crystallizer works most efficiently when full, and when operated continuously.

The overflow from the crystallizer, i.e., of about 50% P₂ O₅ product, may contain a few crystals (Fe/Al complexes). These go substantially back into solution during a subsequent concentration, e.g., to 60%, and do not cause sludge problems.

The pilot plant product was used to fill three tank cars, each containing 8000-10000 gallons of 60% acid specific gravity typically 1.818. Each of the three cars was shipped away from the plant site. About 3-4 weeks was required to fill each car, and another three weeks to ship and unload the car. Sludge in all of these cars was reported as negligible, i.e., about 0.1-0.2% by weight. Furthermore, even this minimal amount was fluid and pumpable, indicating that standard car cleaning equipment would not be required for cars in which the acid of my invention had been shipped.

In this connection it would be common practice for tank cars containing wet process acid, similar to that herein described except not treated by my process, to deposit sludge over the same time period to the extent of about 5%. This sludge deposits in the bottom of the tank car. It cannot be removed conveniently at the point of destination, and generally has no use whatsoever by the purchaser. Customarily the tank car containing the sludge is returned to the production plant, where it is necessary to clean the car using special equipment and hot water or acid to break up the sludge. Great care must be taken in cleaning sludge-containing cars, because phosphoric acid tank cars have a special rubber lining about 1/4 inch thick, which must not be broken during the cleaning operation.

Table 2 following provides analytical data for feed, clarifier, crystallizer, and resulting products as made by the process of this invention. As has been mentioned, feed analysis is variable and may depend on the type of rock available and on preceding process factors. Variables for clarification and crystallizer operation have been mentioned in the test. The data in the Table represent typical clarifier and crystallizer runs.

All products within the product columns show great improvement over comparable prior art products as regards post-precipitation. This is true even at the lower P₂ O₅ grades of acid. A preferred product is the (approximate) 60% P₂ O₅ acid, data for which is given in the second product column.

                                      TABLE 2                                      __________________________________________________________________________     Typical Operating and Pilot Plant Data                                         (See especially FIG. 3)                                                                                    Products of this Invention                                                        60%                                             Clarifier        Crystallizer                                                                              52%                                                                               Acid  Broad Preferred                           Component                                                                               Over-                                                                              Under- Over-                                                                              Under-                                                                             Acid                                                                              Cf. FIG. 3                                                                           operable                                                                             range for                           Wt. % Inlet                                                                             flow                                                                               flow                                                                               Inlet                                                                             flow                                                                               flow                                                                               (1)                                                                               (2)   range 58-60% acid                         __________________________________________________________________________     P.sub.2 O.sub.5                                                                      29.30                                                                             29.30                                                                              22.40                                                                              50.10                                                                             49.40                                                                              46.20                                                                              52.41                                                                             60.00 2.00-63.00                                                                           58.00-60.00                         Solids                                                                               3.00                                                                              0.20                                                                               25.00                                                                              0.80                                                                              0.40                                                                               20.70                                                                              0.07                                                                              0.20  0.00-0.50                                                                            0.00-0.07                           H.sub.2 SO.sub.4                                                                     3.00                                                                              2.50                                                                               12.40                                                                              3.90                                                                              3.60                                                                               6.70                                                                               4.01                                                                              4.50  3.00-6.00                                                                            3.00-6.00                           CaO   0.80                                                                              0.20                                                                               3.70                                                                               0.30                                                                              0.10                                                                               2.40                                                                               0.09                                                                              0.07  0.01-0.10                                                                            0.02-0.25                           SiO.sub.2                                                                            0.70                                                                              0.60                                                                               1.00                                                                               0.40                                                                              0.05                                                                               0.60                                                                               0.12                                                                              0.02  0.00-0.20                                                                            0.00-0.03                           F     1.70                                                                              1.40                                                                               3.80                                                                               0.80                                                                              0.60                                                                               2.20                                                                               0.82                                                                              0.50  0.30-1.50                                                                            0.35-0.65                           Fe.sub.2 O.sub.3                                                                     1.10                                                                              1.10                                                                               1.00                                                                               1.80                                                                              1.50                                                                               3.30                                                                               1.74                                                                              1.90  1.00-5.00                                                                            1.00-3.00                           Al.sub.2 O.sub.3                                                                     0.80                                                                              0.80                                                                               0.70                                                                               1.40                                                                              1.40                                                                               1.40                                                                               1.58                                                                              1.70  1.00-5.00                                                                            1.00-3.00                           Na.sub.2 O                                                                           0.20                                                                              0.10                                                                               1.00                                                                               0.10                                                                              0.05                                                                               0.80                                                                               0.08                                                                              0.07  0.01-0.20                                                                            0.02-0.17                           K.sub.2 O                                                                            0.06                                                                              0.05                                                                               0.30                                                                               0.06                                                                              0.05                                                                               0.25                                                                               0.09                                                                              0.06  0.01-0.15                                                                            0.01-0.12                           MgO   0.30                                                                              0.30                                                                               0.30                                                                               0.50                                                                              0.50                                                                               0.50                                                                               0.62                                                                              0.60  0.30-0.70                                                                            0.30-0.70                           __________________________________________________________________________      (1) Average of 8 runs, using 3 different phosphate rock sources. Solids o      0.07% compares with 5% solids in similar acid not treated by the process       of this invention.                                                             (2) Typical average of several runs in pilot plant of Example 3.         

The use of perlite to make a grade of wet process phosphoric acid substantially free from post-precipitation has been described in my U.S. Pat. No. 3,907,680, in which perlite was used as a filter bed to remove impurities from phosphoric acid by filtration, thereby to inhibit deposition of sludge in the filtered material. Also perlite has been added to wet process phosphoric acid to remove fluorine therefrom as SiF₄ ; see U.S. Pat. No. 2,987,376 to Gloss, Method for Defluorination of Phosphoric Acid. Also the preparation of a wet process phosphoric acid substantially free from post-precipitation is not new. For example, such material can be prepared by simply permitting sludge to deposit from a standing body of wet process acid until such sludge no longer collects. At this point the upper acid will no longer deposit sludge and can be shipped out as sludge-free acid. However, the amount of storage equipment required and the amount of time required make the process uneconomical. So far as I can determine, my process is the first to provide a wet process phosphoric acid with a high mineral content and which is nevertheless free from substantial post-precipitation problems.

If desired perlite or other aluminum silicate material can be added again after, e.g., the first evaporation and/or the second evaporator. In such case the stream would pass to a crystallizer for removal of any additional sludge. Ordinarily, however, plural additions of perlite or other aluminum silicate are not necessary.

Some of my sample jars containing product made by this invention have stayed substantially sludge-free for months.

The product has at least all the uses of merchant grade phosphoric acid. It is especially useful in making liquid fertilizers and as a feed for superphosphoric acid.

PREFERRED EMBODIMENTS

The following sets forth some of the embodiments which I prefer in the operation of my process.

The number 1 filtrate is preferably 22-35% P₂ O₅, and even more preferably 28-32% P₂ O₅. It enters the process at a temperature preferably within the range 30°-65° C., and even more preferably 46°-54° C. The actual temperature of the No. 1 filtrate as used in the pilot plant was about 30° C. If the No. 1 filtrate is colder than the ranges given, it should preferably be steam-heated to bring it up to temperature. The solids content of the No. 1 filtrate may vary typically from 0.5 to 8%. The product that was used in the pilot plant was generally about 3% solids in all cases. The analysis of the No. 1 filtrate can vary quite a bit, but is typically and preferably approximately that given in Table 2, Clarifier Inlet.

The amount of flocculant is preferably 0.02 to 2.0 pound per ton of P₂ O₅ in the No. 1 filtrate; even more preferably this weight ratio is 0.03 to 0.08 pounds per ton of P₂ O₅. Typically, in my pilot plant runs, 0.05 pounds of flocculant was used per ton of P₂ O₅. The flocculant is made up into an aqueous solution prior to addition to the No. 1 filtrate. The concentration of this aqueous solution can vary but is preferably within the range of 0.02 to 1.0 weight percent, of flocculant based on the weight of water; even more preferably the range is 0.04 to 0.1 weight percent; typically in the pilot plant runs the concentration was 0.085 weight percent.

The capacity of the clarifier is preferably 5-20 square feet/GPM; even more preferably this range is 61/2-81/3 square feet/GPM. Specifically, in the pilot plant runs, a clarifier was used having a capacity of 81/3 square feet/GPM. The underflow as a percent of feed to the clarifier is preferably 2 to 20% by volume of the feed; and even more preferably 7 to 9%. The typical ratio as used in the pilot plant was about 8%, based on volume of feed to the clarifier.

As regards addition of perlite or other aluminum silicate material, a preferred range is 1-40 pounds per ton of P₂ O₅. A more preferred range is 5-15 pounds per ton, and in typical pilot plant usage, I used 6.8 pounds of perlite per ton of P₂ O₅.

In the first evaporator, it is preferred to concentrate the initial acid to 42-52% P₂ O₅, and even more preferably 46-50% P₂ O₅. Pilot plant runs were about 48-50%. If a second evaporator is used, the acid is concentrated to a range of 52-63% P₂ O₅ ; and even more preferably 54 to 60% P₂ O₅. The pilot plant generally gave a product about 60% P₂ O₅ in the second evaporator.

The crystallizer is preferably adjusted to give about 10 to 40 weight percent solids in the underflow; even more preferably this range is 20 to 25 percent. In the pilot plant the weight of solids in the underflow averaged about 23.6%. To achieve this the underflow volume based on the feed, is preferably 1-10%, and even more preferably 1-4%. A typical ratio in the pilot plant was about 2%.

My analyses suggest that the solid product of the crystallizer is an undefined conglomerate, mostly of iron, calcium, and aluminum phosphates; calcium sulfate, and fluoride; and with minor amounts of insoluble complex phosphates and/or fluorides of sodium, potassium, strontium, titanium, and magnesium.

A curious difference in the product of this invention and merchant grade phosphoric acid of the prior art is its behavior upon ammoniation to make a "9-27-0" slurry fertilizer. Ordinarily, when merchant grade acid of the prior art is reacted with ammonia to give ammonium phosphates in suspension, e.g., in making 9-27-0 liquid fertilizer slurries, the suspension is black, owing, it is thought, to suspended carbon in the acid. However, when my product is similarly ammoniated, the resulting ammonium phosphate suspension is brown-colored. Furthermore, as regards ammoniations, the products of this invention do not foam, whereas prior art products foam copiously when ammoniated.

Although perlite is the preferred additive, and is exemplified in the above runs, aluminum silicates generally appear to be operable in the process of this invention. (Perlite is an aluminum silicate.) Another aluminum silicate which has been tried is a material obtained as a byproduct in the preparation of sodium fluoride for bauxite refining. It is referred to herein as Aluminum Silicate A. Its analysis is given below, compared to a range typical of perlites. (Values in percent by weight.)

    ______________________________________                                         ALUMINUM                                                                       SILICATE A      CALCULATED     TYPICAL                                         AS REC'D        TO DRY BASIS   PERLITE                                         ______________________________________                                         P.sub.2 O.sub.5                                                                        0.50        0.96           0.01 to 0.12                                CaO     0.07        0.13           0.78 to 3.8                                 SiO.sub.2                                                                              35.66       68.55          67 to 75                                    Fe.sub.2 O.sub.3                                                                       0.00        0.00           0.25 to 3.2                                 Al.sub.2 O.sub.3                                                                       5.70        10.96          10 to 19                                    F       6.56        12.61          --                                          MgO     0.01        0.02           0.01 to 0.70                                Dry Solids                                                                             52.02       100.00         --                                          Na.sub.2 O                                                                             0.05        0.10           1.2 to 4.2                                  K.sub.2 O                                                                              0.02        0.04           2.2 to 6                                    ______________________________________                                    

It can be noted that the SiO₂ and Al₂ O₃ of the Aluminum Silicate A on a dry basis are within the ranges shown for perlite. The Na₂ O and K₂ O, however, are outside of the ranges indicated for perlite.

The following are typical bench scale results when using Aluminum Silicate A compared to perlite, in the process of this invention.

The first column is a control. The second column is perlite and the third column is Aluminum Silicate A adjusted to be equivalent to dry perlite.

All of the following analyses are on product acid.

    ______________________________________                                                                         ALUMINUM                                                             PERLITE.sup.2                                                                            SILICATE.sup.3                                 ACID      FIRST       1/4% of   1/4% dry basis                                 ANALYSIS  CONTROL.sup.1                                                                              the P.sub.2 O.sub.5                                                                      of the P.sub.2 O.sub.5                         ______________________________________                                         P.sub.2 O.sub.5                                                                          59.90       16.90     59.30                                          CaO       0.05        0.07      0.19                                           SiO.sub.2 0.00        0.00      0.00                                           Fe.sub.2 O.sub.3                                                                         2.03        2.13      2.08                                           Al.sub.2 O.sub.3                                                                         2.22        2.28      2.61                                           F         0.69        0.59      0.72                                           H.sub.2 SO.sub.4                                                                         5.47        5.85      5.61                                           MgO       0.59        0.62      0.61                                           Solids    0.91        0.04      0.10                                           Na.sub.2 O                                                                               0.07        0.07      0.05                                           K.sub.2 O 0.09        0.09      0.10                                           ______________________________________                                          .sup.1 Shows layer of sediment on bottom and wall scaling after one month      .sup.2 Thin layer of crystals on bottom; no wall scaling after one month.      .sup.3 Thin layer of crystals on bottom; some wall scaling after one           month.                                                                   

    ______________________________________                                                                         ALUMINUM                                                             PERLITE.sup.2                                                                            SILICATE A.sup.3                               ACID      SECOND      1% of the 1% dry basis                                   ANALYSIS  CONTROL.sup.1                                                                              P.sub.2 O.sub.5                                                                          of the P.sub.2 O.sub.5                         ______________________________________                                         P.sub.2 O.sub.5                                                                          59.70       59.20     60.20                                          CaO       0.05        0.16      0.07                                           SiO.sub.2 0.00        0.00      0.00                                           Fe.sub.2 O.sub.3                                                                         1.45        1.45      1.44                                           Al.sub.2 O.sub.3                                                                         2.03        2.10      2.15                                           F         0.59        0.84      0.61                                           H.sub.2 SO.sub.4                                                                         4.39        4.28      4.42                                           MgO       0.54        0.55      0.54                                           Solids    1.03        0.00      0.00                                           Na.sub.2 O                                                                               0.05        0.04      0.04                                           K.sub.2 O 0.10        0.09      0.10                                           ______________________________________                                          .sup.1 Layers of sediment on bottom and wall scale after one month.            .sup.2 Slight layer on bottom; no scale after one month.                       .sup.3 Slight layer on bottom; slight wall scale after one month.        

In general, by equal substitution of perlite and Aluminum Silicate A, an acceptable product was obtained in the laboratory. However, scaling on the walls of the sample jars was noticeable in the Aluminum Silicate A samples whereas no scaling occurred with the perlite samples.

Aluminum Silicate A offered a problem in that during concentration of the acid in the pilot plant evaporator, scale tended to deposit on the interiors of the evaporator tubes. In one run the tubes plugged on account of scaling. This problem was not encountered with perlite. In a commercial plant this problem would require that the tubes be descaled from time to time. (As a matter of fact, scaling is a common problem in commercial phosphoric acid evaporators, aside from the use of Aluminum Silicate A, and a typical evaporator must be shut down for cleaning after, say, 240 hours of operation. Perlite may be unique in that it causes little or no evaporator scaling.) Another aluminum silicate, kaolin, was tried. Kaolin is a clay, consisting principally of kaolinite, Al₂ O₃.2SiO₂.2H₂ O. In one mode I used kaolin in its natural form. In another mode I calcined the kaolin to drive off the water. Both forms performed about like Aluminum Silicate A, i.e., they both gave fair prevention of post precipitation (though not as good as perlite) and both gave typical scaling in the evaporator.

The results below are bench scale tests with kaolin compared to perlite, at 1% of the P₂ O₅.

    ______________________________________                                                KAOLIN     KAOLIN                                                              CALCINED.sup.1                                                                            AS REC'D  PERLITE                                            ______________________________________                                         P.sub.2 O.sub.3                                                                         60.40        60.00     61.20                                          CaO      0.15         0.16      0.10                                           SiO.sub.2                                                                               0.04         0.03      0.01                                           Fe.sub.2 O.sub.3                                                                        2.67         2.65      2.65                                           Al.sub.2 O.sub.3                                                                        2.46         2.46      2.34                                           F        0.98         1.01      0.88                                           H.sub.2 SO.sub.4                                                                        4.90         4.80      4.70                                           MgO      0.65         0.65      0.66                                           Solids   0.16         0.23      0.04                                           Na.sub.2 O                                                                              0.03         0.04      0.04                                           K.sub.2 O                                                                               0.12         0.12      0.12                                           ______________________________________                                    

Below is an analysis of the kaolin on an as received basis:

    ______________________________________                                         P.sub.2 O.sub.5                                                                         0.00         H.sub.2 SO.sub.4                                                                         0.35                                           CaO      0.36         MgO       0.12                                           SiO.sub.2                                                                               56.44                                                                 Fe.sub.2 O.sub.3                                                                        1.20         K.sub.2 O 0.24                                           Al.sub.2 O.sub.3                                                                        41.71        Free H.sub.2 O                                                                           0.97                                           F        0.01         Comb. H.sub.2 O                                                                          14.36                                          ______________________________________                                    

The above runs with Aluminum Silicate A and kaolin were made in a pilot plant using a procedure like that described in Example 2.

The above disclosure and examples have dealt principally with number one filtrate (22-35% P₂ O₅) as the starting acid. Actually, acid containing 22-46% P₂ O₅ is operable. Such starting acid must be pre-clarified to remove suspended solids, just as was done for number one filtrate. The higher ranges of P₂ O₅ are readily available by evaporating number one filtrate to the desired concentration. All the process steps described for number one filtrate are the same with the more concentrated acids (36-46% P₂ O₅). Accordingly, when using a starting acid in the higher concentrations, say 36-46% P₂ O₅, this still has to be subjected to all the subsequent steps of the process of this invention; viz., it must be further concentrated, following aluminum silicate addition, up to 42-54% P₂ O₅. Thus, when starting with 46% acid, this has to be concentrated to an acid in the range of 47-52% P₂ O₅, and when starting with an acid of 36% P₂ O₅, this is concentrated to 42-52%. In each instance, the concentrated product goes to the crystallizer following which, if desired, it may be further concentrated, all as above described for 22-35% P₂ O₅ acid.

The conclusions in the preceding paragraph are based on work reported below.

In general the number one filtrate was clarified with polymer only, no other additives, then concentrated, then split into two parts, perlite added to one part, and both portions then evaporated to 50%, settled overnight, decanted, and the supernatants evaporated to 60%.

ADDITION OF PERLITE TO 42.9% P₂ O₅ ACID

In this series a sample of number one filtrate was clarified with stock polyacrylamide flocculant solution, decanted, and the supernatant analyzed and found to be as follows:

    ______________________________________                                                    CH2-1230                                                                       1st Day                                                             ______________________________________                                                P.sub.2 O.sub.5                                                                      28.50                                                                    CaO   0.40                                                                     SiO.sub.2                                                                            1.09                                                                     Fe.sub.2 O.sub.3                                                                     0.84                                                                     Al.sub.2 O.sub.3                                                                     0.74                                                                     F     2.29                                                                     H.sub.2 SO.sub.4                                                                     2.20                                                                     MgO   0.37                                                                     Solids                                                                               0.07                                                                     Na.sub.2 O                                                                           0.14                                                                     K.sub.2 O                                                                            0.08                                                              ______________________________________                                    

The above was evaporated to 42.9% P₂ O₅.

    ______________________________________                                                    CH6-1135                                                                       2d Day                                                              ______________________________________                                                P.sub.2 O.sub.5                                                                      42.90                                                                    CaO   0.60                                                                     SiO.sub.2                                                                            1.07                                                                     Fe.sub.2 O.sub.3                                                                     1.55                                                                     Al.sub.2 O.sub.3                                                                     1.36                                                                     F     2.38                                                                     H.sub.2 SO.sub.4                                                                     3.50                                                                     MgO   0.52                                                                     Solids                                                                               1.50                                                                     Na.sub.2 O                                                                           0.19                                                                     K.sub.2 O                                                                            0.12                                                              ______________________________________                                    

This sample was split, one portion evaporated without perlite to 51.3% and the other with perlite to 51.0%. Both allowed to stand overnight, then decanted.

    ______________________________________                                                WITHOUT PERLITE                                                                              WITH PERLITE                                                     CH 7A-1250    CH 7B-1230                                                       3rd day       3rd day                                                   ______________________________________                                         P.sub.2 O.sub.5                                                                         51.30           51.00                                                 CaO      0.61            0.79                                                  SiO.sub.2                                                                               0.42            0.35                                                  Fe.sub.2 O.sub.3                                                                        1.80            1.80                                                  Al.sub.2 O.sub.3                                                                        1.64            1.72                                                  F        1.78            1.63                                                  H.sub.2 SO.sub.4                                                                        4.00            4.10                                                  MgO      0.58            0.58                                                  Solids   1.50            1.71                                                  Na.sub.2 O                                                                              0.22            0.36                                                  K.sub.2 O                                                                               0.13            0.16                                                  ______________________________________                                    

Both samples were further evaporated to 60.6% and 59.8%.

    ______________________________________                                                    CH 9A-1435                                                                              CH 9B-1500                                                            3rd day  3rd day                                                    ______________________________________                                         P.sub.2 O.sub.5                                                                             60.60      59.80                                                  CaO          0.43       0.20                                                   SiO.sub.2    0.00       0.00                                                   Fe.sub.2 O.sub.3                                                                            2.15       2.14                                                   Al.sub.2 O.sub.3                                                                            2.12       2.08                                                   F            1.06       0.95                                                   H.sub.2 SO.sub.4                                                                            4.40       4.10                                                   MgO          0.64       0.65                                                   Solids       0.79       0.00                                                   Na.sub.2 O   0.07       0.06                                                   K.sub.2 O    0.12       0.13                                                   ______________________________________                                    

Visual examination of the above on the 27th day showed over 1/4" layer on the bottom of sample CH 9A-1435 and less than 1/16" on the bottom of CA 9B-1500. This indicates the beneficial effect of perlite.

ADDITION OF PERLITE TO 44.6% P₂ O₅ ACID

Another series of runs was made similar to the one preceding. Analysis of the number one filtrate after clarification showed:

    ______________________________________                                                    CH2-1135                                                                       1st Day                                                             ______________________________________                                                P.sub.2 O.sub.5                                                                      27.80                                                                    CaO   0.39                                                                     SiO.sub.2                                                                            1.12                                                                     Fe.sub.2 O.sub.3                                                                     1.05                                                                     Al.sub.2 O.sub.3                                                                     0.75                                                                     F     2.30                                                                     H.sub.2 SO.sub.4                                                                     2.40                                                                     MgO   0.39                                                                     Solids                                                                               0.00                                                                     Na.sub.2 O                                                                           0.09                                                                     K.sub.2 O                                                                            0.04                                                              ______________________________________                                    

The sample was brought up to 44.6% P₂ O₅ (not decanted at this point).

    ______________________________________                                                    CH6-1930                                                                       2d Day                                                              ______________________________________                                                P.sub.2 O.sub.5                                                                      44.60                                                                    CaO   0.58                                                                     SiO.sub.2                                                                            0.71                                                                     Fe.sub.2 O.sub.3                                                                     1.66                                                                     Al.sub.2 O.sub.3                                                                     1.50                                                                     F     1.98                                                                     H.sub.2 SO.sub.4                                                                     4.10                                                                     MgO   0.57                                                                     Solids                                                                               1.53                                                                     Na.sub.2 O                                                                           0.20                                                                     K.sub.2 O                                                                            0.12                                                              ______________________________________                                    

The sample was then split and perlite added to one portion. After evaporation and standing overnight, then decanted.

    ______________________________________                                                NO PERLITE                                                                               PERLITE (1/2% of P.sub.2 O.sub.5)                                    CH 7A-1543                                                                               CH 7B-1610                                                           3rd Day   3rd Day                                                       ______________________________________                                         P.sub.2 O.sub.5                                                                         49.50       49.90                                                     CaO      0.33        0.27                                                      SiO.sub.2                                                                               0.21        0.28                                                      Fe.sub.2 O.sub.3                                                                        1.80        1.07                                                      Al.sub.2 O.sub.3                                                                        1.52        1.54                                                      F        2.26        1.72                                                      H.sub.2 SO.sub.4                                                                        4.50        4.40                                                      MgO      0.63        0.62                                                      Solids   1.52        0.73                                                      Na.sub.2 O                                                                              0.06        0.05                                                      K.sub.2 O                                                                               0.08        0.06                                                      ______________________________________                                    

These samples were then taken up to 59.8% and 60.6%.

    ______________________________________                                                    CH 9A-1648                                                                              CH 9B-1720                                                            3rd Day  3rd Day                                                    ______________________________________                                         P.sub.2 O.sub.5                                                                             59.80      60.60                                                  CaO          0.49       0.16                                                   SiO.sub.2    0.00       0.00                                                   Fe.sub.2 O.sub.3                                                                            2.24       2.24                                                   Al.sub.2 O.sub.3                                                                            2.06       2.11                                                   F            1.00       0.96                                                   H.sub.2 SO.sub.4                                                                            5.20       4.60                                                   MgO          0.67       0.68                                                   Solids       1.17       0.00                                                   Na.sub.2 O   0.07       0.07                                                   K.sub.2 O    0.11       0.12                                                   ______________________________________                                    

On the 34th day the above showed considerable post precipitation in sample CH 9A-1648, about 1/8" to 1/4" layer in 31/2" total liquid height, whereas CH 9B-1702 showed practically no post precipitation. This is a good example of the beneficial effect of perlite.

OPERATION IN A 70,000 TON/YR. PLANT

The products made in the commercial plant during its initial months of operation (cf. FIG. 4 and description, below) are like those made in the pilot plant (cf. FIGS. 1-3 and description, above) in that both types do not substantially post-precipitate. Aside from this there may be certain differences in the two products. As will be noted from the analyses given herein, the commercial product (in the initial months of operation) is higher in several components than the pilot plant product, typically including solids, CaO, SiO₂, F, and K₂ O. In the initial months the increase in solids may be particularly marked. Solids in the pilot plant product was generally under 0.5%, but in the commercial product may initially average about 1.3%. Despite this increase in solids and other components, the commercial product as first produced undergoes little or no post precipitation. The solids that are present stay largely in suspension, and the small amounts that drop out are soft, "non-setting", and do not interfere with pumping the acid, e.g., from tank cars.

After some months, as the plant reaches optimum conditions, and is adjusted to use preferred conditions as hereinafter described, solids are expected to drop to quite low levels, averaging 0.14 wt.%.

In a commercial operation the products of this invention are best obtained from Florida pebble phosphate rock which is not too high in iron and alumina. Rock in which combined Fe₂ O₃ and Al₂ O₃, divided by P₂ O₅ is about 0.08 or less (e.g., 0.065) is eminently preferred. At higher iron and alumina levels, special treatment is required to attain post-precipitation control, e.g., addition of sulfuric acid to the concentrated acid, as further described in my copending application Ser. No. 54,446, filed July 3, 1979, now U.S. Pat. No. 4,248,846 "Improved Production of Stabilized Wet Process Phosphoric Acid". A high iron and alumina rock might analyze typically P₂ O₅ 32.4, Fe₂ O₃ 1.59, and Al₂ O₃, 1.44%, giving a ratio of (1.59+1.44)/32.4=0.09.

FIG. 4 is a diagrammatical flow sheet of a large scale plant capable of making the products of the instant invention, with a design capacity of 70,000 tons/year. At 71 is shown the head box. This is the initial vessel for receipt of several streams. One such stream is stream 73, from flash cooler 77, which receives recycled slurry via line 75 from the second digester 119. A vacuum line 79 maintains a reduced pressure on the flash cooler. Line 81 feeding into the head box 71 is from clarifier 159, further described below. Line 83 is return acid from the No. 2 filtrate, also described below. Proceeding now with the basic operation of the wet process phosphoric acid plant, phosphate rock is fed via line 87 into rock feeder 89, from which rock exit line 91 delivers metered amounts of phosphate rock into first mix tank 95. Exit line 93 delivers liquid from the head box 71 also into mix tank 95. Sulfuric acid via line 97 and water via line 99 are fed into dilution cooler 101, and this diluted sulfuric acid proceeds via line 103 to two branches, lines 105 and 107, each of which feeds respectively into a mix tank, line 105 feeding into mix tank 95 and line 107 into mix tank 111. The effluent from mix tank 95 exits via line 109 into mix tank 111, and the effluent from mix tank 111 exits via line 113 into first digester 115, from which exit line 117 feeds into second digester 119. Some of the slurry from digester 119 is returned via line 75 to head box 71 as already mentioned. The rest of the effluent from the second digester flows via line 121 into filter feed tank 123, from which the exits at line 125 into the prayon filter, shown here schematically in sections identified simply as 127, 129, and 131. Each section actually undergoes the steps of taking a first filtrate, followed by washing, followed by passing the wash water through a second operation, and the return of the resulting weak acid to the head box. These steps are further described as follows: Effluent line 125 from the filter feed tank proceeds to prayon filter 127, where it is filtered, and the No. 1 filtrate is delivered via line 133 into collection tank 135, from whence the effluent proceeds via line 137, to operations to be subsequently described. Wash water is fed via line 149 through the gypsum filter cake sitting on the filter leaf, as shown at 131, and the effluent waters pass via line 143 as No. 3 filtrate to collection vessel 145. The effluent line from vessel 145, line 147, containing the No. 3 filtrate, passes through another gypsum filter cake, at 129, and the washings are taken via line 139 to form No. 2 filtrate. The No. 2 filtrate is collected in vessel 141, and returned via line 83 to the head box 71 as already described. The filtration sequence is thus filtration through filter 127, which then becomes filter 129 and is washed with the No. 3 filtrate, following which filter No. 129 becomes filter 131, which is washed with plain wash water (typically pond water) to provide No. 3 filtrate used in the preceding step, which preceding step provides No. 2 filtrate which is returned to the head box. Returning now to line 137, No. 1 filtrate, this will proceed to launder mixer 155, where it is mixed with flocculation polymer, prepared as an aqueous solutionin tank 151 and pumped via line 153 to the launder mixer. The effluent line 157 from the launder mixer feeds to clarifier 159, which has overflow 161 and underflow 81. Overflow 161 feeds to a surge tank 167. Effluent from the surge tank 167 is pumped via line 169 to perlite addition tank 170, which discharges to first evaporator (herein "E") 171. Perlite is fed via line 165 to launder-mixer 166, which exits via line 164 to line 169. In evaporator 171 the acid is concentrated up to about 40% P₂ O₅. Effluent proceeds via line 173 to a second evaporator (herein "F") 175, where it is concentrated up to about 48-52% P₂ O₅. Effluent line 177 takes the acid from evaporator 175 via line 177 to crystallizer-settler 179, which has overflow line 181 and underflow line 203 (which may be utilized as described in my copending application Ser. No. 54,446, now U.S. Pat. No. 4,248,846). Overflow line 181 feeds to surge tank 183, and the effluent from the surge tank is taken by line 185 to branches 187 and 189. If the product is not to be concentrated further, commercial product can be taken out of line 187. If the product is to be concentrated further it is taken via line 189 to a third evaporator (herein "G") 191. Effluent line from evaporator 191, namely line 193, is the final product line. Concentration in evaporator 191 typically proceeds to a P₂ O₅ content within the range of 52 to 63%.

The following three tables, 3, 4, and 5 give analyses at various points along the flow sheet of FIG. 4, for three different days, during the initial months of operation.

                                      TABLE 3                                      __________________________________________________________________________                                          "F" Evap-          "G" Evap-                         Feed to                                                                              Clarifier                                                                            Clarifier                                                                             Feed to "E"                                                                           orator                                                                               Crystallizer                                                                          Crystallizer                                                                         orator                      Rock feed,                                                                           Launder,                                                                             Overflow,                                                                            Underflow                                                                             Evaporator                                                                            Product                                                                              Overflow,                                                                             Underflow                                                                            Product                Wt. %                                                                               Line 91                                                                              Line 137                                                                             Line 161                                                                             Line 81                                                                               Line 169                                                                              Line 177                                                                             Line 181                                                                              Line 203                                                                             Line                   __________________________________________________________________________                                                             193                    A.sub.2 O.sub.5                                                                     31.40 29.4  28.9  22.9   28.6   50.6  51.2   44.3  60.8                   CaO  47.3  1.14  0.21  5.55   0.26   0.40  0.01   2.80  0.00                   SiO.sub.2                                                                           8.56  1.00  0.99  0.95   0.92   0.25  0.13   0.91  0.00                   Fe.sub.2 O.sub.3                                                                    0.89  0.86  0.91  0.85   0.89   1.41  1.41   1.26  1.75                   Al.sub.2 O.sub.3                                                                    0.90  0.70  0.75  0.59   0.66   1.39  1.60   1.66  1.70                   F    3.57  2.28  2.22  2.35   2.30   1.80  1.44   4.19  1.08                   H.sub.2 SO.sub.4                                                                    0.73  3.4   2.3   12.1   8.6    4.3   3.2    8.9   3.5                    MgO  0.33  0.32  0.33  0.28   0.33   0.53  0.44   0.71  0.52                   Na.sub.2 O                                                                          0.59  0.20  0.14  0.38   0.17   0.26  0.07   0.90  0.09                   K.sub.2 O                                                                           0.35  0.19  0.14  0.12   0.18   0.35  0.19   0.40  0.14                   Solids                                                                              --    2.3   0.1   18.1   0.3    4.2   1.5    17.5  1.4                    Sp Gr.                                                                              --    1.364 1.348 1.470  1.350  1.694 1.656  1.742 1.820                  __________________________________________________________________________

                                      TABLE 4                                      __________________________________________________________________________                                          "F" Evap-          "G" Evap-                         Feed to                                                                              Clarifier                                                                            Clarifier                                                                             Feed to "E"                                                                           orator                                                                               Crystallizer                                                                          Crystallizer                                                                         orator                      Rock feed,                                                                           Launder,                                                                             Overflow,                                                                            Underflow                                                                             Evaporator,                                                                           Product                                                                              Overflow,                                                                             Underflow                                                                            Product                Wt. %                                                                               Line 91                                                                              Line 137                                                                             Line 161                                                                             Line 81                                                                               Line 169                                                                              Line 177                                                                             Line 181                                                                              Line 203                                                                             Line                   __________________________________________________________________________                                                             193                    P.sub.2 O.sub.5                                                                     30.61 27.6  29.3  23.9   28.9   50.9  51.8   42.4  62.6                   CaO  45.7  1.50  0.38  4.98   0.87   0.90  0.17   4.06  0.26                   SiO.sub.2                                                                           8.92  0.87  0.89  0.96   0.92   0.39  0.21   1.82  0.14                   Fe.sub.2 O.sub.3                                                                    0.92  0.86  0.89  0.89   0.89   1.39  1.65   1.19  1.59                   Al.sub.2 O.sub.3                                                                    0.97  0.63  0.83  0.77   0.67   1.40  1.33   1.80  1.70                   F    3.46  2.14  1.94  2.29   2.04   1.66  1.02   4.72  0.57                   H.sub.2 SO.sub.4                                                                    0.29  1.8   1.7   11.9   2.4    3.3   2.1    9.9   2.7                    MgO  0.37  0.29  0.29  0.28   0.29   0.48  0.42   0.80  0.50                   Na.sub.2 O                                                                          0.59  0.20  0.15  0.34   0.18   0.22  0.04   1.28  0.06                   K.sub.2 O                                                                           0.08  0.08  0.08  0.12   0.09   0.11  0.09   0.38  0.11                   Solids                                                                              --    3.1   0.4   17.4   1.9    3.1   0.7    20.9  0.5                    Sp Gr.                                                                              --    1.344 1.346 1.400  1.346  1.690 1.650  1.812 1.828                  __________________________________________________________________________

                                      TABLE 5                                      __________________________________________________________________________                                          "F" Evap-          "G" Evap-                         Feed to                                                                              Clarifier                                                                            Clarifier                                                                             Feed to "E"                                                                           orator                                                                               Crystallizer                                                                          Crystallizer                                                                         orator                      Rock feed                                                                            Launder,                                                                             Overflow,                                                                            Underflow                                                                             Evaporator,                                                                           Product                                                                              Overflow,                                                                             Underflow                                                                            Product                Wt. %                                                                               Line 91                                                                              Line 137                                                                             Line 161                                                                             Line 81                                                                               Line 169                                                                              Line 177                                                                             Line 181                                                                              Line 203                                                                             Line                   __________________________________________________________________________                                                             193                    P.sub.2 O.sub.5                                                                     31.68 30.3  31.3  17.3   31.2   51.7  50.2   49.4  61.7                   CaO  45.50 1.60  0.30  1.89   0.42   0.54  0.10   1.84  0.13                   SiO.sub.2                                                                           8.88  0.56  0.56  0.85   0.61   0.26  0.11   0.49  0.07                   Fe.sub.2 O.sub.3                                                                    0.75  0.76  0.80  0.56   0.80   1.30  1.35   1.24  1.59                   Al.sub.2 O.sub.3                                                                    1.11  0.84  0.85  0.91   0.74   1.33  1.29   1.92  1.56                   F    3.53  2.39  2.07  3.50   2.09   1.44  0.99   2.85  0.70                   H.sub.2 SO.sub.4                                                                    0.28  3.8   2.3   23.4   2.3    4.0   3.4    6.0   4.0                    MgO  0.31  0.31  0.27  0.47   0.28   0.44  0.39   0.62  0.46                   Na.sub.2 O                                                                          0.54  0.21  0.11  0.78   0.14   0.19  0.05   0.64  0.06                   K.sub.2 O                                                                           0.07  0.09  0.07  0.18   0.08   0.11  0.11   0.16  0.13                   Solids                                                                              --    4.7   0.6   45.9   1.2    2.8   1.0    9.4   1.0                    Sp Gr.                                                                              --    1.400 1.370 1.766  1.362  1.698 1.686  1.748 1.846                  __________________________________________________________________________

                  TABLE 6                                                          ______________________________________                                                         Table 3                                                                               Table 4  Table 5                                        ______________________________________                                         Phosphate rock feed, line 17,                                                                    40       44       40                                         tons/hr.                                                                       No. 1 filtrate, line 67, GPM                                                                     130      180      no data                                    to launder 85                                                                  Perlite, line 95, lbs/hr.                                                                        15       30       60                                         "E" evaporator feed,                                                                             125      150      150                                        line 99, GPM                                                                   Descaler, cc/min. 10       0        0                                          Air to crystallizer 109, psi                                                                     4        2        no                                         data                                                                           "G" evaporator vacuum, inches                                                                    29.2     29.3     29.1                                       "G" evaporator, T, °C. in,                                                                85       89       90                                         line 119                                                                       "G" evaporator, T, °C. out,                                                               89       89       90                                         line 123                                                                       ______________________________________                                    

Table 6 supplements the analytical data given in Tables 3, 4 and 5, and represents plant data taken on or about the same days on which the data in Tables 3, 4, and 5 were taken. Table 6 is concerned largely with feed rates and static readings.

More detailed plant data for about the same period are given in the following Table 7. Here, hourly data were gathered at certain points in the plant, for two sequential days. Ranges for low and high values of the rates are given. For example, in the first column (representing the first day), the phosphate rock feed rate (line 91, FIG. 4), the lowest rate during the day was 43.7 short tons per hour, and the highest, 45.1 tons per hour.

                  TABLE 7                                                          ______________________________________                                         REACTORS                                                                       Rock, TPH          43.7-45.1 39.2-44.6                                         H.sub.2 SO.sub.4, GPM                                                                             83-88     73-88                                             Sp. Gr.            1.330-1.356                                                                              1.322-1.384                                       Acid return, GPM   240-285   210-300                                           Sp Gr.             1.160-1.230                                                                              1.100-1.226                                       Dil. cooling Water, GPM                                                                           26-70     38-88                                             Headbox slurry, deg. C.                                                                           80-83     81-83                                             Agitator slurry, deg. C.                                                                          83-86     81-86                                             Slurry, solids %   47-50     43-49                                             Acid/rock ratio    93-100    90-98                                             Filter cake thickness, In.                                                                        2.5       2.5                                               Filter Warm Water, GPM                                                                            184-192   168-196                                           Filter Warm Water, deg. C.                                                                        40-45     40                                                #1 Filtrate, Spec. gr.                                                                            1.312-1.348                                                                              1.304-1.340                                       #2 Filtrate, Spec. gr.                                                                            1.160-1.230                                                                              1.100-1.240                                       #3 Filtrate, Spec. gr.                                                                            1.028-1.056                                                                              1.026-1.056                                       EVAPORATORS                                                                    Feed rate to E, GPM                                                                               150-160   140-150                                           E effluent, spec. gr                                                                              1.432-1.466                                                                              1.460-1.510                                       F effluent, spec. gr                                                                              1.644-1.680                                                                              1.646-1.694                                       Abs. pressure, inches Hg                                                       E evap.            13.5-18.0 15.0-17.0                                         F evap.            27.5-28.0 27.8                                              E evap, incoming T, deg. C.                                                                       82.4.sup.1                                                                               79.9.sup.1                                        E evap, outgoing T deg. C.                                                                        76.7.sup.1                                                                               74.8.sup.1                                        F evap., incoming T, deg. C.                                                                      82.4.sup.1                                                                               84.4.sup.1                                        F evap., outgoing T, deg. C.                                                                      79.7.sup.1                                                                               79.0.sup.1                                        G effluent, spec. gr.                                                                             1.790-1.822                                                                              1.792-1.820                                       G evap., incoming T, deg. C.                                                                      87.7      89.6.sup.1                                        G evap., outgoing T, deg. C.                                                                      82.2.sup.1                                                                               83.6.sup.1                                        ______________________________________                                          .sup.1 Mean of 25 consecutive hourly entries.                            

As above mentioned, solids content dropped still further in subsequent months as the plant achieved preferred operating conditions.

Preferred embodiments for operating the commercial plant above described (cf. FIG. 4) thus include:

(1) using a 50/50 mix of Hookers Prairie (so-called) (HP) phosphate rock and Bonny Lake (BL) rock (from the Central Florida area). The analyses of these two rocks are typically:

    ______________________________________                                                          HP   BL                                                       ______________________________________                                         P.sub.2 O.sub.5, wt %                                                                             32.4   31.40                                                CaO                46.2   47.3                                                 SiO.sub.2          5.3    8.56                                                 Fe.sub.2 O.sub.3   1.59   0.89                                                 Al.sub.2 O.sub.3   1.44   0.90                                                 F                  3.47   3.57                                                 SO.sub.4 as H.sub.2 SO.sub.4                                                                      1.6    0.73                                                 MgO                0.32   0.33                                                 Na.sub.2 O         0.49   0.59                                                 K.sub.2 O          0.06   0.35                                                 ______________________________________                                    

(2) Perlite use, at the rate 16 lb./ton of P₂ O₅ in the crude acid at the perlite addition vessel.

(3) NaOH addition to clarifier, at the rate of 3 lbs. (Na₂ O equivalent) per ton of P₂ O₅ in the crude acid in the clarifier. (NaOH is preferred to Na₂ CO₃ for addition to the clarifier.)

(4) Sulfuric acid addition to crude phosphoric acid stream to first evaporator, at the rate of 8% H₂ SO₄ based on the weight of P₂ O₅ of the phosphoric acid at this point.

(5) Fe in the rock feed comprises mostly ferric Fe with possibly some ferrous Fe. The ferrous/ferric ratio may appear in the acid (No. 1 filtrate) going to the clarifier as

Ferrous Fe, 1.69 g./liter

Ferric Fe, 8.35 g/liter

Using the above preferred conditions, a series of 35 consecutive runs was carried out (per the drawing) to make 58% P₂ O₅ acid. The average (mean) solids in this series was 0.14%, with a range of 0% to 1.1%. Solids were measured on samples taken from a tank receiving effluent from line 193, FIG. 4. The tank had a capacity equivalent to five days normal production (at 70,000 tons/year), i.e., about 21 tank cars. However, owing to rapid turnover, it generally carried only about four tank cars.

The products of this invention, e.g., those in Table 8, are characterized in that (a) such solids as exist are suspended and nearly non-settling; (b) such solids as exist are soft, free flowing, and non-caking; (c) the solids-containing acid is readily pumped from storage and does not form a cake over outlets, valves, and accessory mechanisms; (d) the acid can be ammoniated without substantial foaming. The products may actually post precipitate a little on standing, thereby picking up another percent, or two, or even more, of solids. Yet substantially all solids, whether initially present or forming as a post precipitate, conform to the aforesaid description a-d. The above criteria apply to acids of this invention in lab samples and/or shipped and stored in conventional equipment and for normal use times, e.g., in tank cars, typically of 200,000 lbs.

As noted from the above description, the acid products of the invention are not subjected to any filtration step beyond the initial filtration to remove gypsum and to provide the No. 1 filtrate as a starting feed. In this sense the products are unfiltered.

In the specification and claims all percentages are by weight unless otherwise stated.

                  TABLE 8                                                          ______________________________________                                         Some Products of This Invention                                                            "High P.sub.2 55; "                                                                       "Low P.sub.2 O.sub.5 "                                                                    Low-high                                     Component   Acid Wt. % Acid Wt. % range, Wt. %                                 ______________________________________                                         P.sub.2 O.sub.5                                                                            57-63      52-55      52-63                                        Solids      0-2        0.5-3      0-3                                          SO.sub.4 as H.sub.2 SO.sub.4                                                               3.5-5      2.5-5.5    2.5-6.0                                      CaO         0.05-1     0.09-0.4     0-1.2                                      SiO.sub.2   0.02-0.15  0.03-0.25    0-0.25                                     F           0.5-1.5    0.9-1.3    .35-1.5                                      Fe.sub.2 O.sub.3                                                                           1.1-2      1-3        .5-3                                         Al.sub.2 O.sub.3                                                                           1.1-2      1-3        .5-3                                         Na.sub.2 O  0.025-0.15  .04-0.09  .02-0.2                                      K.sub.2 O   0.025-0.15 0.01-0.12  0.01-0.2                                     MgO          0.5-0.65  0.35-0.6   0.3-0.7                                      Sum of CaO, SiO.sub.2,                                                         F, Na.sub.2 O, K.sub.2 O, and                                                  MgO         1.1-3.6    1.4-2.8    0.7-4                                        ______________________________________                                    

A particularly valuable product is like that of "High P₂ O₅ " Acid above, except that the P₂ O₅ is 58-61%.

The above analyses are characteristic of products soon after their preparation, e.g., as made on samples taken from the respective evaporator streams or the initial storage vessels for the respective streams. In some cases there will be a modest but harmless increase in the amount of solids after the initial analysis.

In a series of experiments aimed at determining increase in solids on standing, 60% P₂ O₅ acid by my process (made during the initial months of commercial operation) was analyzed as it left the G evaporator 191 (FIG. 4), then stored, and then analyzed again in two weeks. The following results were obtained:

    ______________________________________                                         Run    Initial Solids  Final Solids                                            ______________________________________                                         1      0.3 Wt. %       0.9 Wt. %                                               2      1.5 Wt. %       0.9 Wt. % (a decrease)                                  3      0.7 Wt. %       1.2 Wt. %                                               4      0.2 Wt. %       2.2 Wt. %                                               5      1.2 Wt. %       2.9 Wt. %                                               ______________________________________                                    

The average increase was thus 0.84 wt. percentage points, with the maximum being 2 percentage points. Experience indicates the increase may range from zero (or even less, i.e., a decrease) up to about 3 percentage points. When the "increase/decrease" is less than zero, obviously some of the solids initially present have dissolved. This has been known to happen on occasion, though the reasons are obscure.

Referring to the above Table 8, the "Low P₂ O₅ " Acid (52-55% P₂ O₅) is conveniently made in the G evaporator 191, by simple reduction of heat input and/or increased throughput from the F evaporator 175 via the crystallizer 179 and line 185. In other words, the G evaporator, which ordinarily makes a 60% P₂ O₅ product, is run under conditions of lower concentration.

The problem of post precipitation in prior art 52-54% P₂ O₅ merchant grade acid is bad enough; it is impossible with prior art 60% acid, as the post precipitation in such 60% acid is even worse, and for that reason (prior to my invention) 60% merchant grade acid was not ordinarily available commercially for shipment over any considerable distance or storage for any substantial time.

The tank car cleaning problem with prior art sludge is particularly aggravated in that harsh measures (scraping, chipping) tend to penetrate the rubber lining of the car. This cleaning "fact of life" necessitates great caution and prolonged cleaning times.

Aside from the augmented post-precipitation problems of an ordinary 60% P₂ O₅ merchant grade acid, such product is technically very difficult to make using prior art 50% P₂ O₅ feed acid. The latter tends to boil and entrain in the 60% evaporator. This behavior is quite aside from its post precipitation problems. On the other hand, my 50% product from line 189 from crystallizer 179 (see 177, FIG. 4) is readily concentrated in the "G" evaporator (191, FIG. 4) to 57-63% without boiling or entraining. This is another advantageous yet unobvious characteristic of my 52-55% product (Col. 2, Table 8).

In a series of runs made during the initial months in the above described 70,000 ton/year wet process phosphoric acid plant using this invention, extending over a period of a little over two months, 47 daily samples were taken from "G" evaporator product (line 193), and analyzed for P₂ O₅ and solids (inter alia). The average (mean) of the 47 runs was 59.72% P₂ O₅. A particularly suitable P₂ O₅ range was 59-61%. The values above 61% were 5 runs at 61.5% and one run at 62%. There were 10 runs below 59%, namely two at 56, one at 56.5, two at 57, one at 57.5, three at 58, and one at 58.5. The average (mean) of the 47 runs was 1.3% solids. The three lowest values were 0.5, 0.5, and 0.6%. The three highest values were 2.1, 2.2, and 5%. Sampling and/or analytical errors may have been involved in the 5% report. A range of about 0.8-1.8% solids was typical for a P₂ O₅ range of 59-61%. As above noted, under the stated preferred conditions, the average solids later dropped to 0.14%.

The P₂ O₅ in the final product can be controlled with a fair degree of precision, simply by evaporating more or less water from the product in the final concentration stage. The exact amount of solids and of other components is a consequence of all the preceding steps in the process, and cannot be predetermined with certainty. These values will, however, normally and inherently fall within the ranges here stated, if the process is operated as herein described. For example, the operator cannot "aim" specifically at a product with exactly 0.14% solids. However, proper operation of the process as described will in general result in a product carrying solids of about 0-2%, so that his desired solids percentage will not be far from his actual result.

The reason why the suspended solids do not readily settle out (as they do in ordinary 52-54% P₂ O₅ merchant grade acid) is not entirely clear. One reason is apparently that the solid particles in my product are unusually small, being an estimated 5 microns. Also, they are of relatively low density (about 2). These factors contribute to their enduring suspension, especially in 57-63% acid (density about 1.8).

The advantages of my new phosphoric acid products may be demonstrated with respect to their behavior on shipment in standard phosphoric acid tank cars. See FIG. 5. In FIG. 5 a phosphoric acid tank car is indicated at 215. It will be noted that this tank car slopes gently from the ends toward the center, to sump 217, to which is attached discharge pipe 219, closed by dismountable flange 221, and equipped with valve 223. Tank car 215 is equipped with a dip tube 225, closed by flange 227, rupture diaphragm 229, man hole access 231, and various convenience equipment including ladder 233, and hand rail 235. Flange 227 can be removed for unloading.

Phosphoric acid is corrosive, and will corrode the tank car unless the latter is protected by a coating. Conventionally this coating is a rubber lining, 1/4 inch thick. The coating covers the interior of the tank car. When a tank car of ordinary wet process merchant grade phosphoric acid is shipped, it begins to lay down a deposit of sludge. Owing to the motion of the car, the layer tends to deposit not only on the bottom of the car but also will coat the front and rear ends, as well as the sides. This layer can become quite thick, e.g., up to 36 inches in aggravated cases. This sludge layer is indicated generally at 237 in FIG. 5.

In shipping ordinary merchant grade acid, sludge tends to collect in the sump 217 and the exit pipe 219, and to foul the valve 223. This is in addition to settling on the bottom and sides of the tank car as already described. This fouling gives an immediate problem on attempting to empty the car via pipe 219, since the caked sludge around the exit equipment first has to be broken up. This work typically requires one to two hours. Even after the initial clearances have been made, additional pieces of sludge and cake might flow into the valve and exit pipe, slowing or blocking flow of the acid.

There are two ways to remove acid from a tank car. One way is through the sump out the bottom, as already described. The other way is through the dip tube 225. In order to manage this the cover flange 227 is removed, a conduit is attached, and compressed air is blown in through pipe 229. Again, this procedure requires that the dip tube terminus at the bottom of the tank car be free of cake. If it is not free, it has to be made free. This may be accomplished by the use of an air lance. If that doesn't work, the dip tube may have to be pulled out. These procedures are time-consuming and frustrating.

Neither of the two aforesaid procedures for draining prior art acid from a tank car are effective for removing the layer of sludge deposited on the bottom and sides by ordinary merchant acid. This layer is a hard cake in most cases, and cannot be flushed out easily. Men have to enter the tank car physically with cleaning equipment to dislodge this cake. Aside from the time, effort and inconvenience involved in cleaning out the cake, following removal of the acid from the tank car, there is a severe economic loss represented by the amount of cake that is left. Prior art merchant grade acid typically deposited 5 to 10% of its P₂ O₅ content as sludge, which caked up in the car and was unavailable to deliver to the customer, since the customer pays only for the acid pumped from the car. The cake that is left is for the account of and is the responsibility of the seller. Since phosphoric acid may sell at $250.00 per ton of P₂ O₅, it is evident that the loss per tank car might run $500.00 to $1000.00. This loss is doubly unfortunate, since the sludge has value by reason of its P₂ O₅ content.

The instant products (especially the 58-63% P₂ O₅ products) differ from the prior art products inter alia in their freedom from depositing cake and sludge that prevent recovery of solids. By way of contrast with prior art wet process acids, the acids of this invention can be pumped instantly from tank cars, even after the cars have been in transit or in storage for several weeks. My products do not clog valves or exit lines, and do not substantially settle out in transit and/or storage. The small amount of sediment that does deposit is not of a caking type, and is readily washed out. Practically all solids in my products that are in the products when made and shipped out from the commercial phosphoric acid plant stay suspended in the acid during transit and/or storage and become thus available to the customer. This is, of course, advantageous to the customer, who has the advantage of the P₂ O₅ content of the small amount of suspended solids in my products, and it is likewise economically advantageous to the manufacturer, who does not have to absorb the loss of sludge unrecoverable and unsaleable at the point of destination.

A certain small amount of solids can be tolerated in merchant grade phosphoric acid prepared in accordance with my invention without causing any problem in post precipitation. For example, acid made by my process may on occasion show a solid content as high as 3% in the finished acid as shipped, analyzing 52 to 63% P₂ O₅. However, my pre-treatment operations to prevent post precipitation have the result that most of these solids stay in suspension in the acid. For example, as the acid sits in a tank car for weeks on end, only minimal amounts of solids precipitate. Even these minimal amounts do not drop out as sludge, but rather as a thin covering on the rubber lining of the tank car, readily removed when the car is pumped out.

SUMMARY COMPARISON OF UNLOADING ACID OF THIS INVENTION WITH NORMAL PHOSPHORIC ACID

Unloading a tank car of normal merchant grade phosphoric acid usually requires a considerable period of time just getting the acid to flow. Sludge deposits over the unloading valve act like a plug and must be broken through. Operators will hook air to the valve and blow out the plug if possible. An air lance, down the dip pipe, is used sometimes to clear the valve. With evaporator product it has been necessary to drive a bar up through the valve to get flow started. It is not unusual to spend two hours getting the flow started. If the car has not been cleaned on the last couple of trips, sludge can stop flow and the procedure must be redone. Time required to start flow ranges from thirty minutes to two hours. After flow is started, pumping requires two hours minimum. Washing of cars requires about thirty minutes to set up and four to sixteen hours to wash sludge from cars. Some cars will have four feet of solids in the ends and this is very difficult to remove. The sludge is hard and comes loose in chunks that must be broken up by the acid jet. It is a difficult and time consuming job.

In contrast, acid prepared in accordance with this invention (especially 58-63% P₂ O₅) flows as soon as bottom car valve is open. No lance, air blow or rodding is required. Pumping time has averaged one hour and twenty minutes per car. Cars carrying such acid have required only a simple rinse to remove the inch (approximate) of solids in cars. The preparation time for wash is still required but the actual wash is very small.

Analytical

Except for solids and iron, analyses were made by procedures in "Methods Used and Adopted by the Association of Florida Phosphate Chemists", Fifth Ed., 1970 ("AFPC"). Where more than one method is provided, the selections were as follows:

P, Photometric method (phosphate rock), p. 80.

Aluminum oxide, Atomic absorption method, p. 95.

F, Specific ion electrode method, p. 104A.

Water, Karl Fischer method, p. 127.

Phosphoric acid in phosphoric acid, Total phosphoric acid method, p. 132.

Solids were determined by the following procedure:

Apparatus

a. Vacuum filtration system

b. Drying oven adjusted to 105° C.

c. Gooch crucible, porcelain, with perforated bottom

Reagents

a. Asbestos-Powminco brand or equivalent, special for Gooch crucibles, acid washed. This grade asbestos as received should be thick slurried with water and blended in a Waring blender or equivalent for 10 seconds. The blended slurry should then be placed in an Erlenmeyer flask and washed 3-5 times by adding distilled water and decanting to remove the extremely fine material. The washing is accomplished by thoroughly shaking the blended asbestos with each water wash and then allowing it to settle for 5-10 minutes before decantation of the excess water and fine material.

b. Methanol, absolute

Determination

Prepare the Gooch crucible with a pad of asbestos which filters freely under medium suction. (In lieu of asbetos, glass fibers can be used.) Pad should be thick enough to retain all suspended solids. Wash pad several times with distilled water, finally with methanol, and place in drying oven at 105° C. for a minimum of one hour. Remove from drying oven and place in desiccator until cool. Reject or rework crucibles which visually show evidence of pad separation from crucible walls. Weigh crucible rapidly and record tare weight. Resuspend soilds in sample by shaking thoroughly. Immediately weigh approximately five-ten grams of sample, accurately weighed, into the crucible. Place the crucible and contents on a filter flask and filter with strong vacuum until all of the liquid portion has been filtered through. Wash the solids five times with five ml portions of methanol allowing each wash to filter through completely. Remove crucible from suction and place in the drying oven at 105° C. for a minimum of one hour. Cool in desiccator and weigh rapidly as soon as cool. ##EQU1##

In these analyses, samples are well shaken, then immediately analyzed. Thus, values for P₂ O₅, lime, silica, iron, alumina, etc. include components in both liquid and solid phases.

Ferrous and ferric iron were determined by the following procedure:

Reagents

a. Standard Potassium Dichromate

A 0.1 N solution is made by dissolving 4.904 grams and diluting to one liter with distilled water.

b. Stannous Chloride

Sixty grams of the crystallized salt is dissolved in 600 ml of concentrated HCl and made up to 1 liter with distilled water.

c. Phosphoric Acid-Sulfuric Acid Mixture

150 ml concentrated H₂ SO₄ and 150 ml concentrated H₃ PO₄ are diluted to 1 liter with distilled water.

d. Mercuric Chloride

A saturated solution of HgCl₂ (60-70 grams per liter) is made.

e. Diphenylamine

One gram of diphenylamine is dissolved in 100 ml of H₂ SO₄.

Determination

A 5.00 gram sample of acid is weighed into a 250 ml beaker. About 100 ml of distilled water is added to the beaker. Then 15 ml of the phosphoric-sulfuric acid mixture and 4 drops of diphenylamine are added. The solution is stirred and potassium dichromate is added drop by drop until a deep purple color develops. The milliliters of potassium dichromate is recorded and used in the calculation for ferrous iron.

A total iron is run by weighing out 5.00 grams into a 250 ml beaker, adding 25 ml of 1:1 HCl and about one ml of 2% potassium permanganate solution, placing this on the hot plate and boiling until the color of permanganate is destroyed. The iron is then reduced by adding stannous chloride drop by drop until the solution is colorless. Add one-two drops in excess. The solution is allowed to cool to room temperature. The solution is diluted to 100 ml and 15 ml saturated mercuric chloride is added. Add 15 ml of phosphoric-sulfuric acid mixture and 4 drops of diphenylamine indicator. Potassium dichromate is added drop by drop to the stirred solution until a deep purple end point develops.

Calculation: ##EQU2## 

I claim:
 1. As a new composition of matter, unfiltered wet-process phosphoric acid substantially free of post-precipitation, analyzing:

    ______________________________________                                         Component               Wt. %                                                  ______________________________________                                         P.sub.2 O.sub.5         57-63                                                  Solids                  0-2                                                    SO.sub.4 as H.sub.2 SO.sub.4                                                                           3.5-5                                                  Fe.sub.2 O.sub.3        1.1-2                                                  Al.sub.2 O.sub.3        1.1-2                                                  Sum of CaO, SiO.sub.2 F,                                                       Na.sub.2 O, K.sub.2 O, and MgO                                                                         1.1-3.6                                                ______________________________________                                          said acid characterized in that

(a) the solids if any remain substantially suspended and settle out in only small amounts; (b) the solids if any whether suspended or settled, remain soft, free-flowing, and non-caking; (c) said acid is readily pumped from storage and does not cake over outlets, valves, and accessory mechanisms; and (d) the said acid can be ammoniated without substantial foaming.
 2. Composition according to claim 1 in which the P₂ O₅ is about 58-61%.
 3. Composition according to claim 2 in which the P₂ O₅ is about 60%.
 4. Composition according to claim 3 in which the solids is about 1.3%.
 5. Composition according to claim 2 in which solids is about 0-1.1%.
 6. Composition according to claim 5 in which solids is about 0.14%.
 7. Composition according to claim 2 in which solids is about 0.8-1.8%.
 8. Composition according to claim 1 further analyzing:

    ______________________________________                                         Component            Wt. %                                                     ______________________________________                                         CaO                  0.05-1                                                    SiO.sub.2            0.02-0.15                                                 F                    0.5-1.5                                                   Na.sub.2 O           0.025-0.15                                                K.sub.2 O            0.025-0.15                                                MgO                  0.5-0.65                                                  ______________________________________                                    


9. As a new composition of matter, an unfiltered wet process phosphoric acid product substantially free from post precipitation, and having the following analysis:

    ______________________________________                                                            Wt. %                                                       ______________________________________                                                P.sub.2 O.sub.5                                                                              58-60                                                            Solids        0.0-0.5                                                          H.sub.2 SO.sub.4                                                                             3.0-6.0                                                          CaO           0.01-0.10                                                        SiO.sub.2      0.0-0.03                                                        F             0.35-0.65                                                        Fe.sub.2 O.sub.3                                                                             1.0-3.0                                                          Al.sub.2 O.sub.3                                                                             1.0-3.0                                                          Na.sub.2 O    0.02-0.17                                                        K.sub.2 O     0.01-0.12                                                        MgO           0.3-0.7                                                   ______________________________________                                    

said acid characterized in that (a) the solids if any remain substantially suspended and settle out in only small amounts; (b) the solids if any whether suspended or settled, remain soft, free-flowing, and non-caking; (c) said acid is readily pumped from storage and does not cake over outlets, valves, and accessory mechanisms; and (d) the said acid can be ammoniated without substantial foaming.
 10. Composition of matter in accordance with claim 9, having the following analysis:

    ______________________________________                                                            Wt. %                                                       ______________________________________                                                 P.sub.2 O.sub.5                                                                             60.0                                                              Solids       0.2                                                               H.sub.2 SO.sub.4                                                                            4.5                                                               CaO          0.07                                                              SiO.sub.2    0.02                                                              F            0.5                                                               Fe.sub.2 O.sub.3                                                                            1.9                                                               Al.sub.2 O.sub.3                                                                            1.7                                                               Na.sub.2 O   0.07                                                              K.sub.2 O    0.06                                                              MgO          0.6                                                       ______________________________________                                    


11. Composition according to claim 1 or claim 9, said acid product having been prepared from a starting phosphoric acid which when concentrated tends to substantially post-precipitate in storage, said preparation including the following sequential steps:(a) clarification of the starting acid; (b) addition of aluminum silicate to the clarified acid; (c) concentrating the acid to 48-52% P₂ O₅ ; (d) removal of solids from the concentrated acid; (e) concentrating the acid to 57-63% P₂ O₅.
 12. Composition according to claim 11 in which the acid preparation, after concentration, excludes addition of sulfuric acid.
 13. Composition according to claim 1 or claim 11, said acid product having been prepared from a starting phosphoric acid which when concentrated tends to substantially post-precipitate in storage, said preparation including the following sequential steps:(a) clarification of the starting acid; (b) addition of perlite to the clarified acid; (c) concentrating the acid to 48-52% P₂ O₅ ; (d) removal of solids from the concentrated acid; (e) concentrating the acid to 57-63% P₂ O₅.
 14. Composition according to claim 13 in which the acid preparation, after concentration, excludes addition of sulfuric acid.
 15. A wet process phosphoric acid substantially free from post-precipitation, analyzing:

    ______________________________________                                         Component               Wt. %                                                  ______________________________________                                         P.sub.2 O.sub.5         57-63                                                  Solids                  0-2                                                    SO.sub.4 as H.sub.2 SO.sub.4                                                                           3.5-5                                                  Fe.sub.2 O.sub.3        1.1-2                                                  Al.sub.2 O.sub.3        1.1-2                                                  Sum of CaO, SiO.sub.2 F,                                                       Na.sub.2 O, K.sub.2 O, and MgO                                                                         1.1-3.6                                                ______________________________________                                    

said acid characterized in that (1) the solids if any remain substantially suspended and settle out in only small amounts. (2) the solids if any whether suspended or settled, remain soft, free-flowing, and non-caking; (3) said acid is readily pumped from storage and does not cake over outlets, valves, and accessory mechanisms; and (4) the said acid can be ammoniated without substantial foaming, said acid having beenmade by a method that includes the steps: (a) to a wet process phosphoric acid having a P₂ O₅ content of 22 to 46 weight percent from which the suspended solids have been removed, adding 0.05 to 2.0 weight percent aluminum silicate based on the contained weight of P₂ O₅ in the acid to provide an aluminum silicate acid mix, said aluminum silicate being finely divided so as to be soluble in the phosphoric acid as the concentration of the acid is increased from 22-46 weight percent P₂ O₅ to 48-50 weight percent P₂ O₅ ; (b) concentrating the aluminum silicate acid mix to said 48-50 weight percent P₂ O₅ in an evaporation zone thereby to provide an aluminum silicate/acid solution; (c) transferring the concentrated solution to a crystallizing zone having a product stream nearly free of crystals formed in the said zone; (d) recovering the product stream; and (e) concentrating the product stream to about 57-63% P₂ O₅.
 16. A wet process phosphoric acid substantially free from post-precipitation, analyzing:

    ______________________________________                                         Component               Wt. %                                                  ______________________________________                                         P.sub.2 O.sub.5         57-63                                                  Solids                  0-2                                                    SO.sub.4 as H.sub.2 SO.sub.4                                                                           3.5-5                                                  Fe.sub.2 O.sub.3        1.1-2                                                  Al.sub.2 O.sub.3        1.1-2                                                  Sum of CaO, SiO.sub.2 F,                                                       Na.sub.2 O, K.sub.2 O, and MgO                                                                         1.1-3.6                                                ______________________________________                                    

said acid characterized in that (1) the solids if any remain substantially suspended and settle out in only small amounts; (2) the solids if any whether suspended or settled, remain soft, free-flowing, and non-caking; (3) said acid is readily pumped from storage and does not cake over outlets, valves, and accessory mechanisms; and (4) the said acid can be ammoniated without substantial foaming, said acid havingmade by a method that includes the steps: (a) to a wet process phosphoric acid having a P₂ O₅ content of 22 to 35 weight percent from which the suspended solids have been removed, adding 0.05 to 2.0 weight percent perlite based on the contained weight of P₂ O₅ in the acid to provide a perlite-acid mix, said perlite being finely divided so as to be soluble in the phosphoric acid as the concentration of the acid is increased from 22-35 weight percent P₂ O₅ to 48-50 weight percent P₂ O₅ ; (b) concentrating the perlite-acid mix to said 48-50 weight percent P₂ O₅ in an evaporation zone thereby to provide a perlite/acid solution; (c) transferring the concentrated solution to a crystallizing zone having a product stream containing most of the crystals formed in the said zone; (d) recovering the product stream; and (e) concentrating the product stream to about 57-63% P₂ O₅. 